Replace doc pages with redirects to versioned doc pages.

* Add a new `redirect` Jekyll layout.
* Replace all pages under docs/ with redirects to corresponding page under
  versions/master/.
* Prepend links on Documentation sidebar, including generated navs for the
  Skylark Library and Build Encyclopedia, with prefix for versioned directory.
* Add code to both the internal jekyll-config.sh and external jekyll-tree.sh
  to add redirect pages for the Skylark Library and Build Encyclopedia.
* Bring the branched User Manual doc up to date with latest changes.

--
MOS_MIGRATED_REVID=131568800
diff --git a/src/test/java/com/google/devtools/build/lib/BUILD b/src/test/java/com/google/devtools/build/lib/BUILD
index 0317052..a40ba72 100644
--- a/src/test/java/com/google/devtools/build/lib/BUILD
+++ b/src/test/java/com/google/devtools/build/lib/BUILD
@@ -777,7 +777,7 @@
     size = "medium",
     srcs = ["packages/BazelDocumentationTest.java"],
     data = [
-        "//site:docs/bazel-user-manual.html",
+        "//site:versions/master/docs/bazel-user-manual.html",
     ],
     test_class = "com.google.devtools.build.lib.packages.BazelDocumentationTest",
     deps = [
diff --git a/src/test/java/com/google/devtools/build/lib/packages/BazelDocumentationTest.java b/src/test/java/com/google/devtools/build/lib/packages/BazelDocumentationTest.java
index 70fe645..8167bd2 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/BazelDocumentationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/BazelDocumentationTest.java
@@ -42,7 +42,7 @@
    */
   @Test
   public void testBazelUserManual() throws Exception {
-    String documentationFilePath = "site/docs/bazel-user-manual.html";
+    String documentationFilePath = "site/versions/master/docs/bazel-user-manual.html";
     if (OS.getCurrent() == OS.WINDOWS) {
       documentationFilePath = WindowsTestUtil.getRunfile("io_bazel/" + documentationFilePath);
     }